In conventional FSW welding, the tool leaves an impression in the material, at the end of the weld line, said impression generally being made up of a hole having the same geometry as the pin, this constituting a prohibitive defect which has to be remedied.
In FSW welding, various types of tool can be used:                Conventional tools and tools with a double shoulder, or “bobbin tools”, which leave a hole at the end of welding.        Tools with a retractable pin which can avoid the formation of a hole at the end of welding by carrying out progressive retraction of the pin over a significant distance from the end of welding. The French patent application filed by the applicant and published under the reference FR 2900082 describes notably a method implementing such a tool.        
Welding operations with conventional tools or bobbin tools are disadvantaged by the presence of the abovementioned impression at the end of the weld line, this impression frequently requiring the provision of a guard zone, in which the weld line ends, on the parts to be welded, said guard zone being intended to be eliminated so as to give the assembled parts their final dimensions and shape. Such a welding method thus leads to the execution of repeat machining of the welded parts and necessarily to the production of scrap, thereby increasing the assembly costs.
Furthermore, repairing welding defects that can appear along the weld line is not easy to carry out, and so sometimes some assemblies have to be scrapped because it is not possible to carry out satisfactory reworking.
For its part, welding with a retractable pin is more tricky to carry out. The tool used notably has to have a pin with a virtually cylindrical geometry. Since the welding operation also comprises an operation of progressive retraction of the pin at the end of the weld line, the tool employed is necessarily more complex in its design and is also more fragile.
It should also be noted that repairing welding defects along the weld line, holes or fissures, necessarily results in an underthickness of material in the region of the repair.
With regard more precisely to the repair or repeat of poorly executed welds at certain points on the weld line, regardless of the welding method used, it is also known to use friction plug welding methods that consist mainly in introducing an addition of material at the flaw, said addition of material forming a plug, and in friction welding this material to already assembled parts. This technology requires both the removal of the welding tool and repeat machining (or resurfacing) of the two faces of the weld. Moreover, it is necessary to have a specific appliance for implementing such a method.